Modern, futuristic optical trunk traffic networks have to satisfy stringent requirements in terms of capacity, flexibility, reliability and transparency. These requirements for a transport network are optimally satisfied when data are transmitted and switched using the optical frequency division multiplex method. In the case of the optical frequency-division multiplex method (wavelength-division multiplex--WDM), a plurality of transmission channels are combined on one fiber and are separated from one another by their optical carrier frequencies (wavelengths) which differ from one another by several 100 GHz. The maximum number of possible channels in this case limited by the amplification bandwidth of optical fiber amplifiers.
Optical cross-connects (OCC) are provided for semi-permanent and blocking-free coupling of optical channels. Such optical cross-connects, as a rule, have both a frequency switching stage and a space division switching stage.
ICC'93 Conf.Rec. Vol.3/3, 1300 . . . 1307, FIG. 10, in this context discloses a WDM switching arrangement having wavelength demultiplexers on the input side which in each case split the incoming optical signal on the associated input fiber on the basis of wavelengths, a downstream space division switching arrangement (space switch), downstream adjustable wavelength converters which convert the respectively incoming wavelength to the respective outgoing wavelength, and wavelength multipliers on the output side which combine the wavelengths supplied to them.
ntz 46(1993)1, pages 16 . . . 21, FIGS. 13 and 14 discloses WDM switching arrangements in which wavelength demultiplexers (1/N in FIG. 13; unnamed in FIG. 14) are likewise provided on the input side, wavelength multiplexers (N/1 in FIG. 13; unnamed in FIG. 14) are provided on the output side, and wavelength converters (frequency stages in FIG. 13; filter and frequency converters in FIG. 14) are provided for wavelength conversion to the respective outgoing wavelength, with an intermediate space division switching arrangement (space stage in FIG. 13; fiber switch in FIG. 14) is also provided.
In such arrangements of optical (de)multiplixers and space division switching arrangement the same optical frequency (wavelength) may possibly always be applied to each input of the space division switching arrangement, in which case, with the same frequency allocation of the individual optical waveguides in each case to the same frequency division multiplex of M optical carrier frequencies, the optical frequency of an input of the space division switching arrangement is repeated every M inputs.
The object of the space division switching arrangement is to connect the inputs to the outputs without any blocking, that is to say to make it possible to switch a path through the space division switching arrangement, in every load case, between any given free input and any given free output.
One possible architecture for a space division switching arrangement having N inputs and N outputs is a combination of in each case N 1xN tree structures at the N inputs and N outputs with a link network (shuffle network) between the tree structures of the inputs and outputs; in this case, each tree structure can be formed by a pyramid of 1x2 switches (see, for example, JP-A-61194408, JP-A-62020493).
Technical implementations of optical 1x2 switches in fact have only a limited amount of crosstalk attenuation: some of the respective signal also passes to that output which is not currently selected, which has the effect of crosstalk between one signal path and another signal path in the space division switching arrangement. The crosstalk attenuation of currently available optical switches based on semiconductors is still unsatisfactory, and a space division switching arrangement which is formed from such switches therefore does not per se satisfy the system requirements. In this case, crosstalk between two channels at the same optical frequency is particularly critical while, in the case of a crosstalk signal at a different optical frequency, additional crosstalk attenuation can be achieved to a sufficient extent by means of filters in the multiplexer stage downstream of the space division switching arrangement outputs.
For comparatively enhanced crosstalk suppression, it is possible (according to DE-A1-4 432 728), in an optical 1xN switching matrix with a tree structure and having an optical input/output and a number N of optical outputs/inputs, comprising
an optical waveguide structure which connects the input/output with each output/input and comprises optical waveguides which branch like a tree at junction points from the input/output in the direction of outputs/inputs, and PA1 in each case one optical changeover switch per junction point for selectively changing over between waveguides which branch off from these junction points, PA1 two matrix rows of in each case N optical 1xN switching matrices, each lxN switching matrix having in each case one optical input/output and in each case N optical outputs/inputs, and PA1 an optical switching network having two rows of connections, each comprising NxN optical connections, each of which is used as an optical input and/or output, it being possible to connect each connection in a row of connections optically to each connection in the other row of connections, PA1 the total of NxN optical outputs/inputs of the N optical 1xN switching matrices in each matrix row are connected in parallel to the NxN optical connections of in each case one row of connections, and PA1 the total of N optical inputs/outputs of the N optical 1xN switching matrices in each matrix row form the N inputs and/or N outputs of the NxN switching matrix,
to assign to the outputs/inputs in each case one optical gate switch for selectively optically releasing and blocking this output/input as a function of a switching state of the changeover switch at a junction point from which a branching waveguide is connected to this output/input,
in an optical NxN switching matrix having a tree structure with a number N of optical inputs and N optical outputs, comprising
the optical 1xN switching matrices may be 1xN switching matrices designed in the indicated manner.
In this case, the changeover switches and gate switches are expediently 1x2 switches with two switched-on states, in which the light is passed essentially via in each case one of the two switching paths, the respective switched-on path, and a greatly attenuated element of the light, at most, is inadvertently coupled over to the respective other switching path, and with a third state, in which the light experiences the same attenuation on both switching paths and, to this extent, signals are not "switched-through" on either of the two paths. Such 1x2 switches having more than two switching states are, for example (known from B. Acklin, M. Schienle, B. Weiss, L. Stoll, G. Muller "Novel optical switches based on carrier injection in three and five waveguide couplers: TIC and SIC", Electronics Letters, 30(1994)3, 217) TIC switches or else other digital optical switches with a third switching state in which the light experiences the same attenuation on both switching paths. In this case, in each 1xN switching matrix, only those changeover switches and that gate switch via which the intended light path passes are then switched to the corresponding switched-on state and, in addition, the further gate switch which is connected to the changeover switch that is connected to this gate switch is switched to a switching state which leads to an optical sink; all the other changeover switches and gate switches are in the third switching state (DE-A1-4 432 728).
In order to suppress k-th order crosstalk in a multistage optical NxN space division switching arrangement with input-specific optical splitters and output-specific output pyramids of 1x2 switches which may have two switched-on states, (according to EP-A1-0 353 871) these switch pyramids, which each have N inputs, of log.sub.2 N pyramid stages which are intrinsically sufficient for N inputs may also be extended to (k+log.sub.2 N) pyramid stages, only N pyramid inputs being connected, however, to corresponding outputs of N optical splitters and the other pyramid inputs remaining unconnected, and it being possible to switch all 1x2 switches only jointly in each pyramid stage; quite a number (but not all in any case) of the 1x2 switches which are not included in a connecting path may in this case be switched to a switching state of increased crosstalk attenuation.
In fact, in addition to increased crosstalk attenuation, the insertion of additional switching stages into the tree structure also results in an increased insertion loss. However, in addition to the crosstalk attenuation, the insertion loss is a second critical variable of a space division switching arrangement, which it is necessary to optimize.
The invention now indicates a different means of achieving increased crosstalk attenuation, to be precise without also having to accept increased insertion loss at the same time.